1. Field
The presently disclosed subject matter relates to deep-ultraviolet light source used for various applications including disinfection, sterilization and water purification, or in semiconductor manufacturing processes and so on.
2. Description or the Related Art
Generally, deep-ultraviolet, light sources of an about 200 to 260 nm wavelength range have been broadly used as disinfection light sources, sterilization light sources and water purification light sources, or in semiconductor manufacturing processes.
A first prior art deep-ultraviolet light source is a low pressure mercury discharge apparatus using a hot cathode (see: JP5-54857, JP5-217552 & JP6-96609). That is, an anode electrode and a cathode electrode are inserted at ends of a glass discharge tube, and noble gas such as argon (Ar) or neon (Ne) in a low pressure state is sealed in the glass discharge tube. Then, when an about 50 kHz AC voltage is applied between the anode electrode and the cathode electrode, electrons collide with noble gas which also collides with mercury. As a result, mercury is excited to start discharge. In this discharge, a 254 nm wavelength deep-ultraviolet light is emitted from the excited mercury.
In the above-described first prior art deep-ultraviolet light source, however, since harmful mercury is included, it is not preferable in view of eco-efficiency. Also, since the ultraviolet emission intensity depends upon the vapor pressure of mercury which is low, the ultraviolet emission intensity is low. Further, the ultraviolet emission intensity depends upon the environmental temperature. In this case, when the environmental temperature is very low, the ultraviolet, emission intensity is extremely low. On the other hand, when the environmental temperature is higher than 60° C. the ultraviolet emission intensity is also low. Therefore, the optimum environmental temperature range is very small, i.e., from room temperature to 60° C. Further, the rising time period of the ultraviolet emission intensity is long. Still further, the ultraviolet emission efficiency is low. Finally, since the drive voltage is high, the electromagnetic noise is large.
A second prior art deep-ultraviolet light source is a low pressure mercury discharge apparatus using a cold cathode (see: JP2006-12586). This second prior art deep-ultraviolet light source has an advantage in that the ultraviolet emission efficiency is larger than that of the first prior art deep-ultraviolet light source.
However, the above-described second prior art deep-ultraviolet light source still has the same disadvantages as in the first prior art deep-ultraviolet light source except for the ultraviolet emission efficiency.
A third prior art deep-ultraviolet light source is a high pressure mercury discharge apparatus using a hot cathode (see: US Patent Application Publication No. 2009/0184644A1 & JP2007-5182369). That is, noble gas including mercury in a high-pressure state is sealed in a glass discharge tube to enhance the ultraviolet emission intensity.
In the above described third prior art deep-ultraviolet light source, obtained ultraviolet emission intensity is high due to the high pressure of mercury gas; however, the ultraviolet emission efficiency is low because the conversion efficiency from the input electric power to the ultraviolet emission energy is low. Also, the lifetime would be shortened to about 1000 hours. Further, since a circuit for generating a high voltage discharge is required, this deep-ultraviolet light source is high in cost. Still further, in the same way as in the first prior art deep-ultraviolet light source, since harmful mercury is included, it is not preferable in view of eco-efficiency, and since the drive voltage is high, the electromagnetic noise is large.
A fourth prior art deep-ultraviolet light source excites, i.e., pumps a wide band gap semiconductor, i.e., hexagonal boron nitride (hBN) powder including no harmful mercury by an electron beam to emit a 225 nm wavelength deep-ultraviolet light from the excited hBN powder (see: Kenji Watanabe et al., “Far-ultraviolet plane emission handheld device based on hexagonal boron nitride”, Nature Photonics, Vol. 3, 591, October 2009).
In the above-described fourth prior art deep-ultraviolet light source, however, since the self-absorption of light is large and the rate of radiation recombination is small, the ultraviolet emission efficiency is low.
A fifth prior art deep-ultraviolet light source excites, i.e., pumps a wide band gap semiconductor, i.e., AlxGa1-xN/AlN multiple quantum well (MQW) layer including no harmuful mercury by an electron beam to emit a 240 wavelength deep-ultraviolet light from the excited MQW layer (see: Takao Oto et al., “100 mW deep-ultraviolet emission from aluminium-nitride-based quantum wells pumped by an electron beam”, Nature photonics, Vol. 4, 767, September 2010).
More concretely, an AlN layer is formed on a sapphire (0001) substrate, and an Al0.69Ga0.31N (1 nm thick well)/AlN (15 nm thick barrier) MQW layer is grown on the AlN layer. This MQW layer suppresses the self-absorption of light by the Al0.69Ga0.31N well layers to enhance the ultraviolet emission efficiency.
The above-described fifth prior art deep-ultraviolet light source, however, has the following problems.
Firstly, the MQW layer pumped by art electron beam generates a harmful X-ray in addition to the deep-ultraviolet light.
Secondly, since the MQW layer has poor conductivity, irradiation of the MQW layer with an electron beam for even a short time period would electrostatically destroy the MQW layer.
Thirdly, the ultraviolet emission efficiency is still low.
Fourthly, an electron emission source for generating an electron beam may be a thermal electron emission source such as a tungsten (W) filament or a cold-cathode electron emission source such as carbon nanotubes (CNTs), however, such an electron emission source has a short lifetime such as 1000 hours. As a result, this deep-ultraviolet light source has a short lifetime. Also, since the thermal electron emission source requires heating power, the ultraviolet emission efficiency would be low. On the other hand, since the cold-cathode electron emission source requires a high vacuum sealing, the manufacturing cost would become high.